The present invention relates to plant breeding. More particularly, the invention relates to the production of haploids and doubled haploids (whether at the whole plant or seed level)--preferably induced by a haploid inducing gene--and to the selection and/or screening therefor.
The mutant gene "indeterminate gametophyte" (ig), when present in the female parent results in offspring with a high proportion of defective seeds and a greatly enhanced frequency of haploids of both maternal and paternal origin. The ratio of maternal to paternal types is about 1:2. In crosses of maize plants with igig females to four standard inbreds it has been shown that the frequency of androgenesis ranges from about 0.5 to 2 percent with one androgenetic plant in 10 being diploid. However, the influence of the paternal donor is much stronger than hitherto thought. Thus, the average frequency of androgenesis due to ig has been shown to be as little as only 0.03%, with some genotypes perhaps reaching 1.5%. Recently, it has been suggested that placing the ig gene in an early maturing maize genotype, such as the inbred line Co220, should make the production of haploids easier whilst maintaining a similar or higher level of haploid induction. Even so, use of this procedure for large scale production of haploids in practical plant breeding requires an efficient marker system that permits discrimination between haploids and spontaneously doubled haploids of paternal origin on the one hand, and sexual diploids and maternal haploids and doubled haploids on the other hand.
The most reliable marker system hitherto used, at least in respect of maize, is a Purple Embryo Marker stock (PEM) of the genotype b p1 A C R.sup.nj:cu du pr P.sup.wr, originally developed to detect maternal haploids, but used also in connection with ig-induced haploid detection. The critical gene in this system is the R-allele. R.sup.nj:cu du, which in combination with the dominant pigment-conditioning genes A and C causes red or purple pigmentation of the aleurone, primarily on the crown portion of the kernel, and a deep purple pigmentation in the embryo. Any colorless-seed maize stock can function as the haploid donor parent provided it is not homozygous RR and the lack of color is not due to a dominant pigment inhibitor. For the detection of maternal haploids the PEM stock is used as the pollinator whilst for the detection of ig-induced androgenetic haploids a PEM-ig stock is used as the seed parent in crosses with the donor line or breeders stock. With both systems the desired haploids have a white embryo and colored aleurone. While the PEM system permits efficient selection of maternal haploids in the dry kernels, the high proportion of defective and small size kernels in igig females hampers seed selection of androgenetic haploids.